In these years, optical discs are widely used as means for recording and reproducing data such as video data, voice data, and computer data. For example, high density recording optical discs called Blu-ray™ Disc have been put to practical use. With further high density recording optical discs, in order to read out information from one surface side with use of an optical pickup device, a ray bundle needs to be condensed to be focused on a track on a recording surface of interest (in-focus position or optimum focusing position), and in addition the condensed light spot needs to be right on a track of interest. Hence, focusing and tracking servo control is performed to make the light spot be right on a recording portion of a recording layer of an optical disc.
The servo control uses servo error signals (push-pull signals such as a focus error signal and a tracking error signal), in which noise due to external disturbance is mixed causing the offset of the signals, hence presenting a problem. Further noise reduction is required of pickup devices.
For example, a technique is known in which an operation is performed on the light distribution of the far field in an optical pickup device to have the offset of the focus error signal cancelled out. (Refer to Patent Document 1).
The conventional art disclosed in Patent Document 1 is for reducing the offset of the tracking error signal that occurs due to variation in the track pitch of guide tracks.
The embodiment 6 recited in Patent Document 1, paragraphs 0130 to 0133 describes that a beam dividing element 61 having areas 61a to 61d of a four-type divided pattern as shown in FIG. 1 is used in a detection optical system of an optical pickup device to divide return light from an optical disc and allow most of the light to pass through to be a zeroth-order non-diffracted light beam 710 while the areas 61a to 61d diffract the rest into beams 71a to 71d, that in a photo detector 36 having photoreceptor sections 36a to 36h as shown in FIG. 2, the photoreceptor sections 36a to 36d receive the beam 710, and the photoreceptor sections 36g, 36e, 36f, 36h receive the beams 71a, 71b, 71c, 71d respectively, and that the photoreceptor sections 36a to 36h output current signals 136a to 136h corresponding to their respective amounts of received light, operations being performed on them to obtain servo error signals. In the conventional art of the embodiment 6, the beams 71b, 71c (first-order light beams) passing through the areas 61b, 61c are received by the photoreceptor sections 36e, 36f separate and away from the photoreceptor sections 36a to 36d for the main ray bundle (zeroth-order light beam), and operations are performed on their outputs.
In the embodiment 23 recited in Patent Document 1, paragraphs 0292 to 0303, using a polarization hologram element 501 that is divided into six areas 520a, 520b, 521a, 521b, 521c, 521d as shown in FIG. 3, the return light is directed to a four-division photo detector 30 having four photoreceptor sections 30a to 30d as shown in FIG. 4. As described in Patent Document 1, paragraph 0297, light having passed through the area 521a becomes a light beam 541d, which is received by the photoreceptor section 30c; light having passed through the area 521b becomes a light beam 541c, which is received by the photoreceptor section 30b; light having passed through the area 521c becomes a light beam 541b, which is received by the photoreceptor section 30d; and light having passed through the area 521d becomes a light beam 541a, which is received by the photoreceptor section 30a, positioned in a diagonal relationship to each other.
The description of the paragraph 0302 of Patent Document 1, embodiment 23, recites that because the polarization hologram element also moves at the same time that the objective lens moves, the division lines do not relatively move, resulting in the amount of offset being small, that in order to reduce the amount of this offset, the regions around the center of the light beam are exchanged in position and detected, thereby reducing the influence of the shift of the light amount distribution, and that by exchanging the regions in the diagonal relationship, an FE signal of an astigmatism method and a TE signal of a phase differential method are not greatly affected.    Patent Document 1: Japanese Patent Application Laid-Open Publication No. 2004-281026